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Study looks at effects of lack of diversity in Chinook salmon releases

A study examining Chinook salmon releases in the Salish Sea has found that the sizes of salmon released, and the times of year they are released, have become increasingly uniform over the last five decades. And this lack of diversity could have potential consequences on salmon health and on other species that live in this body of water shared by the United States and Canada, according to a recent study.

The primary author of the study, Benjamin Nelson, is a Seattle-based consultant and PhD candidate at the University of British Columbia in Canada.

“The most interesting result that we found was that the diversity of hatchery Chinook salmon that are released from hatcheries in the Salish Sea has declined quite a bit since the 1970s,” said Nelson. “In the ’70s, hatcheries tended to release Chinook salmon throughout a very wide range of release times. They used to release fish from February through August and they used to release them a quite a few different sizes, all the way from fry to fully smolted young salmon.”

The study has shown that since the 1970s, hatchery practices in the area have homogenized. They now tend to release most of their fish within a two- or three-week window in May and that they release fish that are very similar in size.

Nelson said this is not necessarily a bad thing. However, having this loss of diversity could have consequences in the long-term. Harbor seals, for example, are known to be fairly intelligent. It’s not farfetched, Nelson said, to believe that they could be learning where and when large hatchery releases occur. A smaller number of predators could really be taking advantage of high-density releases of hatchery fish.

On the other hand, large “pulses” of hatchery fish could swamp predators and provide a buffer on predation of wild fish. That is why Nelson is calling for further research and collaboration between scientists and hatcheries, in order to determine the full extent and nature of these impacts.

“We control so few aspects of the large marine ecosystem and hatcheries are one of the few things that we can control with a fair degree of precision in terms of how salmon interact with the ecosystem,” he said. “We really have the chance to be proactive in designing experiments to actually get at some of these questions.”

Early efforts toward that type of collaboration are already in the works, said Nelson. Seattle-based non-profit Long Live The Kings is working on bringing parties together to find opportunities for collaboration and information sharing.

The study was originally focused on the much-publicized challenges facing killer whales in the Salish Sea. With much speculation on how prey availability may have impacted their numbers, Nelson wanted to examine the populations of the whales’ primary prey – Chinook salmon.

“It was originally supposed to be a broad look at Chinook salmon populations along the entire West coast,” he said. “But since so many people are interested in the Salish sea populations of Chinook salmon, we chose to start there for the potential implications for killer whales, but also because there are hundreds of Chinook salmon hatcheries in the Salish Sea. It’s pretty unique.”